Documentation improvements

[tyt2y3]

We've addressed a few problems as mentioned on https://cprimozic.net/notes/posts/trying-out-sea-orm/ "Trying Out sea-orm"

1. Regarding "I don't know how to create a db enum", https://www.sea-ql.org/SeaORM/docs/generate-entity/enumeration/#native-database-enum more instructions for using Postgres Enum 933ad35
2. Regarding "the schema syntax is too verbose", a design on possibly less verbose migration syntax Less verbose migration · Issue #2084
3. Regarding "I want to use SQL in migrations", https://www.sea-ql.org/SeaORM/docs/migration/writing-migration/#using-raw-sql more emphasis on using SQL and explaining the benefits of code-based migrations f3ab541

Thank you for your suggestions, and we are constantly looking to improve the library / documentation.